FRIDAY, APRIL 29
■ Mercury-Pleiades conjunction. As the stars come out, look low in the west-northwest for little Mercury close to the Pleiades. They're hardly more than 1° apart. Bring binoculars; even the brightest Pleiades stars are only 3rd and 4th magnitude. Mercury will remain near the Pleiades for the next couple of evenings.
■ Venus-Jupiter conjunction. Set your alarm to get up early Saturday morning and look low in the southeast about 60 to 45 minutes before your sunrise time. There will be Venus and Jupiter, the two brightest objects after the Sun and Moon, spectacularly close at ½° apart. They'll be lined up more or less horizontally.
Think photo opportunity. Pose them over some interesting foreground. A tripod will help.
And, look off to their upper right for much-fainter Mars and Saturn. Mars is 16° from the bright pair. Saturn is about the same distance farther beyond Mars.
SATURDAY, APRIL 30
■ Face north just after nightfall, look very high, and you'll find the Pointers, the end stars of the Big Dipper's bowl, on the meridian pointing toward Polaris straight down below. From the Pointers to Polaris is about three fists at arm's length.
Whenever the Pointers point straight down, Vega is rising low in the northeast, Leo walks horizontally high across the south, and the Arch of Spring (see tomorrow's entry) fills the high west.
■ In Sunday's early dawn, Venus and Jupiter will be only a trace farther apart than they were this morning, now with Jupiter to Venus's upper right.
■ A partial eclipse of the Sun occurs today for the southeastern Pacific and the southern cone of South America. Details.
■ New Moon (exact at 4:30 p.m. EDT).
SUNDAY, MAY 1
■ Look west-southwest as the stars come out in twilight. There's Orion tilting down in his spring orientation, with his belt horizontal, getting lower every day. The belt points left toward Sirius and right toward Aldebaran and, farther on, Mercury near the Pleiades.
■ Use binoculars for that to help with the Pleiades — and with the very thin, one-day old thin crescent Moon down below them! Look very early for the Moon, even before the stars come out; the Moon sets while twilight is still in progress. If you spot it, note the time and find how much time has passed since new Moon, which was at 4:30 p.m. Eastern Daylight Time April 30th. Is this your youngest-ever young-Moon sighting? Not many people have ever seen a crescent Moon less than 30 hours old, unless they were carefully looking for it.
■ As twilight deepens, look very high above Orion for Pollux and Castor, lined up roughly horizontally. They form the top of the huge Arch of Spring.
Lower left of Pollux and Castor, Procyon marks the Arch's left end.
Look farther to the lower right from Pollux and Castor for 2nd-magnitude Menkalinan (Beta Aurigae) and then brilliant Capella, the Arch's right end.
MONDAY, MAY 2
■ Now the crescent Moon, two days old, is much more easily seen hanging upper left of Mercury as shown below. Mercury has faded to magnitude +0.9, nearly matching Aldebaran father to the Moon's left. But notice that Mercury doesn't twinkle as much.
■ Although it's May now, wintry Sirius still twinkles very low in the west-southwest in twilight. It sets soon after. How much longer into the spring can you keep Sirius in view? In other words, what will be its date of "heliacal setting" as seen by you?
The farther south you are, the later this date will be. That's because Sirius is south of the celestial equator. Northerners will have the latest heliacal setting date for Capella in another month or two, since Capella is at a northern declination.
TUESDAY, MAY 3
■ Arcturus is the brightest star high in the east these evenings, shining pale yellow-orange. Look for Spica, paler blue-white, lower right of Arcturus by about three fists at arm's length.
To the right of Spica by half that distance is the distinctive four-star constellation of Corvus, the springtime Crow.
■ Back to Arcturus. It forms the pointy end of the long, narrow Kite asterism formed by the brightest stars of Bo
WEDNESDAY, MAY 4
■ The crescent Moon after dark hangs at the trailing foot of the Castor stick-figure. Once again, as it has done every month for a while now, the Moon forms a nearly isosceles triangle with the horn-tip stars of Taurus: Beta and fainter Zeta Tauri. Beta is about 8° lower right of the Moon; Zeta is more directly below the Moon.
THURSDAY, MAY 5
■ These spring evenings, the long, dim sea serpent Hydra snakes far across the southern sky. Find Hydra's head, a rather dim asterism about the width of your thumb at arm's length, in the southwest. It's lower right of Regulus by about two fists at arm's length. Also, a line from Castor through Pollux points to it about 2½ fists away.
Hydra's tail stretches all the way to Libra rising in the southeast. Hydra's star pattern, from forehead to tail-tip, is 95° long. That's more than a quarter of the way around the celestial sphere.
FRIDAY, MAY 6
■ Three zero-magnitude stars shine after dark in May: Arcturus high in the southeast, Vega much lower in the northeast, and Capella in the northwest. They appear so bright because each is at least 60 times as luminous as the Sun, and because they're all relatively nearby: 37, 25, and 42 light-years from us, respectively.
SATURDAY, MAY 7
■ A gigantic asterism you may not know about is the Great Diamond. It stands some 50° tall and extends across five constellations. It's now upright in the southeast to south after dusk.
Start with Spica, its bottom. Upper left from Spica is bright Arcturus. Almost as far upper right from Arcturus is fainter Cor Caroli, 3rd magnitude. The same distance lower right from there is Denebola, the 2nd-magnitude tailtip of Leo. And then back to Spica. Robert H. Baker may have been the first to name the Great Diamond, in his 1954 book When the Stars Come Out.
The bottom three of these stars, the brightest three, form a nearly perfect equilateral triangle. We can call this the "Spring Triangle" to parallel to those of summer and winter. The first to name it such was probably the late Sky & Telescope columnist George Lovi, writing in the March 1974 issue, but it didn't really catch on. So let's try again. (Here's to you, George!)
■ And if you have a dark sky, or binoculars, look halfway from Cor Caroli to Denebola for the very large, sparse Coma Berenices star cluster. It spans some 4°, about the size of a ping-pong ball held at arm's length.
This Week's Planet Roundup
Mercury peeks through the fading twilight low in the west-northwest, fainter every day! On April 29th and 30th, when it's passing the Pleiades, Mercury is still magnitude +0.4 or +0.5. But as quickly as May 4th it's down to +1.2, only half as bright. 1
Venus and Jupiter, magnitudes –4.1 and –2.1, respectively, are the two "Morning Stars" shining together low in the east-southeast as dawn brightens. On the mornings of April 30th and May 1st, they're passing each other just ½° apart at the times of dawn for the Americas.
Their actual conjunction happens around 19 hours UT April 30th, when the two planets will be ¼° apart. Around that time they will be in dawn view for the western Pacific Rim.
Each morning thereafter, Jupiter moves farther to Venus's upper right.
Mars and Saturn glimmer well to the upper right of Jupiter and Venus. They're vastly fainter, identical now at magnitude +0.9. Mars, however, is more orange than Saturn, which is pale yellow.
Each morning they're a little farther from the Venus-Jupiter pair and from each other. The whole line of four is 33° long on April 30th and lengthens to 41° by May 7th.
Uranus is hidden behind the glare of the Sun.
Neptune is in the background of Jupiter, but at magnitude 7.9 it's unobservable in the dawn glow.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time, EDT, is Universal Time minus 4 hours. (Universal Time is also called UT, UTC, GMT or Z time.)
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read How to Use a Star Chart with a Telescope. (It applies just as much to charts on your phone or tablet as to charts on paper.)
You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."
Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.
"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
— Carl Sagan, 1996
"Facts are stubborn things."
— John Adams, 1770
1 . Okay, you've asked how we do that. Why does Mercury fading 0.75 magnitude mean it fades by half? How do you convert a magnitude difference to a brightness difference?
Since 1856, the stellar magnitude system has been precisely defined so that 5 magnitudes is exactly a 100-times difference in brightness. So, one magnitude is a change in brightness of the fifth root of 100. Which is 2.512 for all practical purposes.
Here's the formula to use: If △m is the magnitude difference, then
brightness difference = 2.512△m
...which is just a few taps on your scientific calculator.
Some amateurs learn the basic magnitude intervals by heart: